The PTH/PTHrP receptor mediates the endocrine actions of PTH on calcium homeostasis and the paracrine/autocrine actions of PTHrP on embryonic development and cellular differentiation. Gene knock-out studies have shown that the PTH/PTHrP receptor is essential for life and that it plays an important role in the normal development of bone. In situ hybridization demonstrated abundant PTH/PTHrP receptor transcript in osteoblasts, as well as in proliferating chrondocytes. Molecular cloning of the PTH/PTHrP receptor gene revealed that this gene has several unique features, which include three 5' non-coding exons that are transcribed, from two distinct promoters, which are used in a tissue-specific manner, resulting in multiple tissue-specific alternatively spliced forms of the PTH/PTHrP transcript. The PTH/PTHrP receptor gene must contain information that directs this pattern of temporal- and tissue-specific expression. This proposal will characterize the regulatory DNA sequences that determine tissue-specific expression of the PTH/PTHrP receptor gene in osteoblasts and chondrocytes, in vitro, using cell line models of osteoblasts and chondrocytes (Specific Aim 1) and in vivo, using transgenic mice (Specific Aim 3) and the nuclear proteins that mediate effects of regulatory sequences (Specific Aim 2). Achievement of the goals of this project will provide an understanding of how the PTH/PTHrP receptor is expressed in a tissue-specific manner. Characterization of the molecular mechanisms regulating PTH receptor gene expression will help define mechanisms of osteoblast and chondrocyte differentiation, maturation and developments.